1. Field of the Invention
The invention relates to the reduction of peak-to-average ratio (PAR) for single-carrier and multi-carrier modulation schemes in digital subscriber lines.
2. Background Information
There are many technologies which impose high bandwidth applications over existing copper wire infrastructure intended for plain old telephones (POTS) communication, such as digital subscriber line (DSL) which comes in multiple variations such as asymmetric digital subscriber line (ADSL), high bit-rate digital subscriber line (HDSL), integrated services digital network (ISDN) digital subscriber line (IDSL), symmetric digital subscriber line (SDSL), Rate Adaptive Digital Subscriber Line (RADSL) and very high bit-rate digital subscriber line (VDSL), collectively known as xDSL. ADSL allows users a higher data rate downstream (i.e., to the customer) than upstream (i.e., to the service provider).
High-bandwidth systems, including DSL systems use single-carrier modulation as well as multi-carrier modulation schemes. Both DSL and other high-bandwidth systems such as wireless use modulation schemes such as Carrier-less Amplitude and Phase modulation (CAP) and Discrete Multi-tone (DMT) for wired media and Orthogonal Frequency Division Multiplexing (OFDM) for wireless communication. One advantage of such schemes is that they are suited for high-bandwidth application of 2 Mbps or higher upstream (subscriber to provider) and up to 8 Mbps downstream (provider to subscriber). Quadrature Amplitude Modulation (QAM) utilizes quadrature keying to encode more information on the same frequency by employing waves in the same frequency shifted by 90°, which can be thought of as sine and cosine waves of the same frequency. Since the sine and cosine waves are orthogonal, data can be encoded in the amplitudes of the sine and cosine waves. Therefore, twice as many bits can be sent over a single frequency using the quadrature keying. QAM modulation has been used in voice-band modem specifications, including the V.34.
CAP is similar to QAM. For transmission in each direction, CAP systems use two carriers of identical frequency above the 4 KHz voice band, one shifted 90° relative to the other. CAP also uses a constellation to encode bits at the transmitter and decode bits at the receiver. A constellation encoder maps a bit pattern of a known length to a sinusoid wave of a specified magnitude and phase. Conceptually, a sinusoidal wave can be viewed to be in one-to-one correspondence with a complex number where the phase of the sinusoidal is the argument (angle) of the complex number, and the magnitude of the sinusoidal wave is the magnitude of the complex number, which in turn can be represented as a point on a real-imaginary plane. Points on the real-imaginary plane can have bit patterns associated with them, and this is referred to as a constellation and is known to one of ordinary skill in the art.
DMT modulation, sometimes called OFDM, builds on some of the ideas of QAM but, unlike QAM, it uses more than one constellation encoder where each encoder receives a set of bits that are encoded and outputs sinusoid waves of varying magnitudes and phases. However, different frequencies are used for each constellation encoder. The outputs from these different encoders are summed together and sent over a single channel for each direction of transmission. For example, common DMT systems divide the spectrum above the 4-kHz voice frequency band into 256 narrow channels called bins (sometimes referred to as tones, DMT tones or sub-channels). These bins are 4.3125 kHz wide. The waveforms in each bin are completely separable from one another. In order to maintain separability, the frequencies of the sinusoidal used in each bin should be multiples of a common frequency known as the fundamental frequency and in addition the symbol period τ, must be a multiple of the period of the fundamental frequency or a multiple thereof. A sinusoid is often represented by a complex number. The association is based on the fact that every sinusoid can be represented as the real part of the function aejωt, where a is the complex number and ω is the frequency of the sinusoid. In accordance with the constellation encoder, the value of a0 is determined by the data to be encoded and the constellation used. Suppose all the bins use frequencies that are a multiple of a fundamental frequency ω0. Then over N bins, the waveform to be encoded is Σn=1, . . . , N anejnω0t, which is an equation easily implemented using an inverse Fast Fourier Transform (IFFT).
From a circuit standpoint, and in relation to discrete multi-tone modulation, the prior art shown in FIG. 1, is a transmitter side of a DMT transceiver. The transmitter accepts serial data which is then converted from serial to parallel form and to provide M signals, n0 . . . nM via a serial to parallel converter 10 (SP). The value of M depend on the standard adopted; for example, ADSL uses 256 tones (M=256), ADSL2+ uses 512 tones (M=512) and VDSL uses even more. The sequences are then passed on to a symbol-mapper 20 where each bit is assigned or mapped into one of N-complex (QAM) multi-level sub-symbols. The M symbols are complex-valued and are fed into an IFFT 30 which provides 2×M output real samples by taking the complex conjugates of the M samples. The parallel outputs of the IFFT are applied to parallel to serial converter 40 to provide a serial output signal. The output of parallel to serial converter 40 is applied to cyclic prefix block 40 which helps to make a channel circular so that equalization can occur more easily in the frequency-domain. The output of the cyclic prefix block is then upsampled and interpolated by up-sampler 60 and interpolator 70. The output is processed by a digital-to-analog converter (DAC) 80 which converts the discrete time signal into a continuous time signal.
High amplitude peaks in the composite time signal occur when the signals from the different tones add constructively. Compared to the average signal power, the instantaneous power of these peaks is high, and consequently, so is the PAR. These large peaks require a large dynamic range of the DAC and analog front end (AFE) which results in inefficient amplifiers with excessive power dissipation and expensive transceivers. To overcome the drawbacks of the high PAR, many solutions and techniques have been proposed, one of which is tone reservation method in which a pre-selected number of tones are set aside for PAR reduction. The signal transmitted in these tones is subtracted from the data signal thus reducing the PAR, but at a cost of increased complexity at the transmitter.
As an example of how different tones can add constructively, a simple example of a DMT system comprising 4 bins at frequencies 1, 2, 3 & 4 is shown. FIG. 2A shows a time-domain realization of the symbol (1,1,0,0). For the purposes of example here, the graphs show voltage plotted against time, but the y-axis could be any form of signal bearing quantity. The resultant form has a distinctive peak at time index 0. This peak is not necessarily characteristic of other symbols; for example, the symbol (1,j,0,0) does not exhibit such a characteristic peak as shown in FIG. 2B. Suppose the bin at frequency 4 is a reserved tone, the addition of the fourth tone at a power level of ¼ shows a reduction in the peak of the signal in FIG. 2A as shown in FIG. 2C. The receiver of this signal will nonetheless only interpret tones at frequencies 1, 2 and 3 for extracting data from the transmitter.
The use of reserve tones in PAR reduction poses many challenges including a tradeoff between amount of PAR reduction, data rate loss, complexity of implementation, standard compliance and interoperability. Accordingly, various needs exist in the industry to address the aforementioned deficiencies and inadequacies.